The present invention generally relates to microelectronic assemblies, and more specifically it relates to semiconductor chip packages having light sensitive or light emitting chips.
The semiconductor chip packaging industry is a highly competitive business in which the packaging companies are waging an ongoing battle to improve the reliability and cost of packaged chips. Moreover, the electronics industry continues to demand that chip packages take up less space within electronic components. As such, it is desirable that the overall size of the packaged chips be reduced so that the same circuitry fits into smaller areas thereby allowing for more portability (size, weight, etc.) for the resulting finished electronic product and/or allowing for an increase in a product""s processing power without also increasing its size.
One type of semiconductor chip package includes packages which incorporate light sensitive semiconductor devices such as integrated circuits. When packaging such light sensitive devices (referred to herein as xe2x80x9cIC""sxe2x80x9d or xe2x80x9cchipsxe2x80x9d), it is necessary to allow for the passage of visible or near visible light, i.e. ultraviolet (xe2x80x9cUVxe2x80x9d) or infrared (xe2x80x9cIRxe2x80x9d) light, to the surface of the packaged chip. For example, ultraviolet-erasable programmable read-only memories (xe2x80x9cUV EPROMSxe2x80x9d) have traditionally been manufactured using a standard lead frame type package, as shown in FIG. 1. Typically in this type of package, a cavity is molded (as by an injection molding operation) around a lead frame and the back surface of the chip is attached to a paddle on the lead frame. The chip contacts on the face surface of the chip are then wire bonded to respective leads on the lead frame. This allows the chip assembly to be enclosed on three sides. The fourth side of the assembly is then fitted with a transparent lid made of glass or quartz so that the chip is physically protected but still capable of receiving light through the lid.
In a similar fashion, when packaging certain light emitting chips such as light emitting diodes (LED""s), it is desirable to allow for the passage of visible or near visible light from a surface of the packaged chip. When manufacturing light emitting diodes, the LED""s are commonly created on leadframe like structures. As shown in FIG. 2, an electrically conductive back surface of the chip is typically attached to an electrically conductive base on a first lead and chip contacts on a face surface of the chip are wire bonded to corresponding second leads. The base of the first lead typically has a notch within which the chip sits. The assembly is then encapsulated by using an injection molding technique. The encapsulant may form a lens on the top of the LED package to aid in the focusing of the light being emitted from the chip.
U.S. Pat. No. 4,890,383 to Lumbard et al. discloses another technique for packaging LED""s which use a supporting frame or substrate as part of the package structure. As set forth in the ""383 patent, the substrate comprises an electrically insulating material such as synthetic plastic. The top surface of the substrate includes a conductive pattern which defines a land area and a connection pad. A light emitting diode is mounted on the land area so that its terminal underneath is electrically and mechanically connected to the land area. The upper side of the light emitting diode is provided with a terminal which is electrically conductive and connected with the connection pad via a bonding wire.
A second conductive pattern of highly conductive material such as copper is deposited onto the rear surface of the substrate. This second conductive pattern defines a first terminal pad and a second terminal pad. The land area on the top side of the substrate is provided with an extension which is electrically connected to the terminal pad on the under side via a plated through groove having a semicircular cross-section. Similarly, the connection pad on the upper side of the substrate is provided with an extension which is electrically connected to the terminal pad on the under side via a plated through groove, which is preferably identical to the plated through groove. In this manner the two terminal pads serve as external terminals for the light emitting diode which mechanically secures modular component during surface mounting of the component. The assembly is provided with a transparent covering for protective purposes. Thus the light emitting diode and its electrical contacts including the bonding wire are sealed and encapsulated in the covering. The covering is made from clear or diffused epoxy, which provides optical characteristics.
Despite these and other efforts in the art, still further improvements in interconnection technology would be desirable. The chip packages manufactured in accordance with the prior art methods described above typically consume greater areas on a printed circuit board or within an electronic component than might otherwise be required. In addition, these particular prior art embodiments are also relatively complex and expensive.
In one embodiment of the present invention, a method of making a microelectronic package having an optoelectronic element, such as a light sensitive or light emitting semiconductor chip, includes providing a sacrificial layer having a first surface and one or more conductive pad areas. One or more conductive pads and a base may be selectively formed on the first surface of the sacrificial layer, preferably by using photolithographic techniques. In certain embodiments, the base may include a conductive material, such as an electrically conductive material. The base and pads are preferably formed using electroplating techniques, whereby the base is located within a central region defined by the conductive pads. After the base and the one or more conductive pads have been formed, the optoelectronic element is provided. The optoelectronic element may have a front face including a plurality of contacts and an optical area for receiving incoming light and a rear surface. The rear surface of the optoelectronic element may then be juxtaposed and attached to the top surface of the base by preferably using an adhesive such as a thermally conductive adhesive. Next, the contacts may be electrically interconnected with the one or more conductive pads on the sacrificial layer using wire bonding techniques. In one preferred embodiment, the wire bonding step includes providing conductive wires having first and second ends, bonding the first ends of the wires to the contacts and bonding the second ends of the wires to the conductive pads. Other elements which may be used to interconnect the contacts and the conductive pads include electroformed beam leads and tape-automated bonding leads (TAB leads). Next, a curable and at least partially transparent encapsulant is provided over the first surface of the sacrificial layer so as to encapsulate the base, the optoelectronic element and the one or more conductive pads and the encapsulant is cured. As used herein, the term xe2x80x9cat least partially transparentxe2x80x9d means a material that allows any amount of light to pass therethrough and includes the terms light transmissive, transparent and/or translucent. In certain preferred embodiments this term means a material transparent to one or more desired wavelengths of light. For example, the material may be transparent to certain wavelengths of light and opaque to other wavelengths of light. After the curing step the sacrificial layer is at least partially removed so as to leave the base and the one or more conductive pads exposed and/or accessible at a bottom surface of the encapsulant. The bottom surface of the encapsulant preferably defines the bottom of the package. In certain preferred embodiments the base and the one or more conductive pads protrude slightly beyond the bottom surface of the encapsulant so that the package may be readily interconnected with an external circuit element such as a PCB. In other preferred embodiments, a base is not used to make the package and the rear surface of the optoelectronic element is attached directly to the first surface of the sacrificial layer during fabrication of the package so that after the sacrificial layer is removed the rear surface of the optoelectronic element is exposed and/or accessible at the bottom of the package. As such, the rear surface of the chip may be directly connected to an external element, such as a heat sink or a PCB. In still other embodiments, the base and/or the one or more conductive pads may be formed from the sacrificial layer.
The method described above may also include the step of placing a lid at a top surface of the encapsulant so that the lid overlies the front face of the optoelectronic element. The lid is preferably transparent or at least partially transparent and is preferably selected from the group consisting of glass, quartz and rigid polymeric materials. In certain preferred embodiments, the lid is formed separately from the package and is attached to the top surface of the encapsulant. In other embodiments the lid may be integrally formed with the top surface of the encapsulant. The lid may include a coating for filtering out certain frequencies of light. The lid may also include a lens for focusing incoming light onto the optical area on the front face of the optoelectronic element. The lens may be of any type, such as a Fresnel-type lens or a telescopic-type lens. In certain preferred embodiments, the top surface of the lid, which is remote from the front face of the optoelectronic element, is provided with a connector. The connector preferably secures an end of a light transmitting cable on the top surface of the lid so as to focus light from the light transmitting cable through the transparent lid and onto the optical area on the front face of the optoelectronic element. The connector may be integrally formed with the lid and may include a cavity for receiving and securing the end of the light transmitting cable.
In other preferred embodiments, a plurality of microelectronic packages may be manufactured simultaneously on the same sacrificial layer using the techniques described above. In these embodiments after the packages have been formed, individual packages are provided by severing the sacrificial layer to provide individual microelectronic packages. In other embodiments, the separating step provides a plurality of multichip modules whereby each module includes two or more of the optoelectronic elements.
In a further embodiment of the present invention, a method of making a microelectronic package includes the steps of providing an electrically conductive sheet and forming a conductive base having a cavity formed therein and at least one conductive pad adjacent the conductive base, the cavity having a light reflective surface. Portions of the conductive sheet may be removed using photolithographic and etching techniques. In other embodiments the conductive pads and conductive base may be formed using a die or stamping machine. The cavity is preferably a depression formed into the first surface of the electrically conductive sheet and may include a bottom portion and side walls extending therefrom. The bottom portion and the side walls of the cavity and the at least one conductive pad may be covered with a layer of a non-oxidizing electrically conductive material, such as silver or gold. Although both silver and gold may be used as effective electrically conductive layers, silver is more preferred because it is relatively more reflective than gold. In other embodiments, the electrically conductive material may also be provided over the second surface of the conductive sheet at locations in alignment with the conductive pad and the cavity. An optoelectronic element, such as a light emitting chip or light emitting diode (LED), having a front face including at least one contact and a rear surface having at least one contact is provided and the rear surface of the optoelectronic element is attached to the base so as to position the optoelectronic element in the cavity and electrically interconnect the optoelectronic element and the conductive base. Next, the at least one contact on the front face of the optoelectronic element may be electrically interconnected with the at least one conductive pad such as by using the wire bonding techniques described above. A curable and at least partially transparent or translucent encapsulant may then be provided over the first surface of the conductive sheet so as to encapsulate the at least one conductive pad, the conductive base and the optoelectronic element. The encapsulant is then cured. In preferred embodiments the encapsulant can be cured by using heat, UV light, a two-part reacting mixture or curing at room temperature. After the curing step, additional portions of the conductive sheet may be selectively removed so as to electrically isolate the conductive base from the at least one conductive pad. The assembly may then be electrically interconnected with an external circuit element by connecting the conductive base and the at least one conductive pad to contacts on the external circuit element.
In certain preferred embodiments of the present invention the encapsulant has different levels of transparency or translucency. For example, in one particular preferred embodiment the encapsulant overlying the optical area of the optoelectronic element is more transparent than the encapsulant overlying the periphery thereof. The encapsulant may also include a compliant material whereby the gradient of compliancy varies throughout the package. In one particular embodiment the encapsulant adjacent the chip contacts is more compliant than the encapsulant adjacent the at least one conductive pads. This particular feature enables the conductive wires or leads adjacent the contacts to readily flex and move as the light sensitive chip or light emitting chip expands and contracts during operation. In still other preferred embodiments the encapsulant includes a color tinted portion for changing the color of the emitted light as perceived by an observer. For example, the light emitted from the optoelectronic element may be red and the encapsulant may have a blue tint so that the emitted light as perceived by an observer is purple.
Certain preferred embodiments of the present invention provide near xe2x80x9cchip sizexe2x80x9d microelectronic packages having light sensitive or light emitting chips whereby the overall size of the package is not much larger than the size of the chip itself. Thus, the final packages have a smaller footprint and will take up less space on a circuit board or within an electronic component which enables the overall size of the final electronic product to be reduced. This is a dramatic advance over prior art packages which teach using bulky substrates which generally dwarf the size of the chip so as to create packages which are much larger than the chip being packaged, thereby wasting a significant amount of space within an electronic device. The present invention also provides methods for mass producing reliable and economical chip packages.
In yet another embodiment of the present invention, a method of making a multichip module having a plurality of optoelectronic elements includes the steps of providing a substrate having a dielectric layer disposed between a top conductive layer and a bottom conductive layer and selectively removing portions of the bottom conductive layer to form an array of conductive pads. Portions of the top conductive layer and the dielectric layer are then selectively removed to form an array of openings extending through the top conductive layer and the dielectric layer, whereby the array of openings are in alignment with the conductive pads. A plurality of optoelectronic elements, such as light emitting semiconductor chips, are then provided over the top conductive layer and electrically interconnected with the top conductive layer. Each optoelectronic element preferably includes a front face having a contact and a rear surface having a contact, whereby the rear surface of each optoelectronic element is preferably attached to the top conductive layer. The rear surfaces may be attached by applying an electrically conductive adhesive over the top conductive layer and abutting the rear surfaces of the optoelectronic elements against the electrically conductive adhesive. Each optoelectronic element may also be electrically interconnected with one of the conductive pads such as by using flexible conductive leads or wires to interconnect the contacts on the front face of the optoelectronic elements and the conductive pads. A curable and at least partially transparent encapsulant is then provided over the top conductive layer and the plurality of optoelectronic elements and the encapsulant is cured. A transparent lid which overlies the optoelectronic elements may be attached over a top surface of the encapsulant layer.
The foregoing and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.